Sean Lee is the Dallas Cowboys' leading tackler by a large margin, but they may not have their steady middle linebacker for a while following an injury in the third quarter on Sunday versus the Carolina Panthers.

UPDATE: Wednesday, October 24 at 11:08 a.m. ET by Brandon Galvin

The Cowboys have reportedly signed Ernie Sims to replace Sean Lee, who will be placed on injured reserve, according to ESPNDallas.com's Todd Archer:

If he does require season-ending surgery, it's a major blow to the Cowboys' stellar defense. After Lee's 58 tackles, Bruce Carter holds the team's second-highest total with 32.

Losing Lee would damage the consistency of Dallas' front seven, allowing teams to have more room to run up front.

He also has one interception and a forced fumble this season. Without a single sack, he lacks game-breaking explosiveness, but that doesn't negate his instinctual read-and-react ability and cover skills.

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Because Dallas runs a 3-4 as its base defense, Lee's loss definitely does not have to be paralyzing for the entire defensive unit, especially if Carter steps up in his absence. Dan Connor, Lee's replacement who had 75 tackles for the Panthers last season, had two tackles once Lee left the game on Sunday.

Connor should be fine if Lee is out on a long-term basis, but someone like Orie Lemon may not be. Nonetheless, he will definitely see an uptick in his playing time if Lee is put on the shelf.

The 26-year-old Lee, Dallas' second-round pick out of Penn State in 2010, is incredibly important to Dallas' new identity on defense. He's tough, and he makes the right play most of the time. The Cowboys are not especially strong up the middle, with Lee as the main stalwart against the run.

The Cowboys are hoping that things aren't as bad as they initially look, but we will have to wait and see.